Faces of people who are from another ethnicity

A note for Lesson no. 3, "Little Brains Wire Themselves to Their World," in Seven and a Half Lessons About the Brain by Lisa Feldman Barrett.
Some context from page 56 is:

...people tend to live around people of the same ethnicity, so babies are often not exposed to a wide array of facial features.

What is this?

Scientists call this the "other-race" face effect. See tuning and pruning.